import java.util.Scanner;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        // 注意 hasNext 和 hasNextLine 的区别
        int n = in.nextInt();
        int m = in.nextInt();
        int a = in.nextInt();
        int b = in.nextInt();
        int maxOne = Math.min(n / 2,m);
        long values = 0;
        for (int i = 0; i <= maxOne; i++) {
            int remainingApple = n - i * 2;
            int remainingPeach = m - i;
            if(remainingApple < 0 || remainingPeach < 0){
                continue;
            }
            int y = Math.min(remainingApple,remainingPeach / 2);
            long total = (long) a * i +(long) b * y;
            if(total > values){
                values = total;
            }
        }
        System.out.println(values);
    }
}